This invention relates to a compact heat exchanger and/or fluid mixing means which incorporates a series of plates having apertures which define a plurality of passages through which fluid may flow.
Compact heat exchangers are characterised by their high xe2x80x9carea densityxe2x80x9d which means that they have a high ratio of heat transfer surface to heat exchanger volume. Area density is typically greater than 300 m2/m3. and may be more than 700 m2/m3. Such heat exchangers are typically used to cool (or heat) process fluids.
One well known but expensive to manufacture type of heat exchanger is the so-called tube and shell heat exchanger. Essentially such heat exchangers consist of an exterior tubular shell through which run a number of longitudinally-extending smaller diameter tubes carrying one or more fluids. Other fluids, with which heat is to be exchanged, typically pass transversely across the heat exchanger such that heat is exchanged through the tube walls. A large number of tubes may be needed and they each have to be individually and accurately fixed/secured into a header plate at each end of the shell. In each case holes need to be drilled in the header plates very accurately to locate the tubes. High quality tested tubing then needs to be assembled into the plates and brazed or welded or mechanically-expanded into position. As the tubes are reduced in diameter to increase surfaces available for heat transfer and hence performance/compactness, the more difficult and expensive such configurations become to manufacture.
A second known type of heat exchanger is the so-called primary plate/secondary plate type exchanger in which a stack of plates is assembled, the stack having primary plates which directly separate two different fluid streams and secondary plates between adjacent primary plates. The secondary plates act as fins which add to the strength of structure and may be provided with perforations to provide additional flow paths for the fluids. The plates are usually bonded together by brazing but this may have the disadvantage of affecting the physical properties of the plates in the brazed regions or may introduce into the system, by means of the braze material, a potentially less satisfactory structure in terms of strength and corrosion resistance. It has been proposed to bond the plates together by diffusion bonding but a satisfactory construction that can withstand the high pressures involved has not been achieved and the fins may buckle during the bonding process.
It is an object of the present invention to provide an improved construction of this second type of heat exchanger which can be satisfactorily made by, for example, diffusion bonding or by brazing. It also aims to provide a heat exchanger construction which can also be readily adapted for use as a fluid mixing means, e.g. it can be used as a chemical reactor in which fluids which are to react together are mixed. Thus, where a reaction is exothermic, the invention may provide a means whereby the exothermic heat of reaction may be removed efficiently or, alternatively, it may be used to supply heat to an endothermic reaction. The products of the invention are also useful as fuel reformers and gas clean-up units associated with fuel cell technology.
Accordingly the present invention provides a heat exchanger or fluid mixing means comprising a bonded stack of plates, the stack comprising at least one group of plates, the group comprising one or more perforated plates sandwiched between a pair of primary separator plates, each perforated plate having perforations arranged in rows across the plate in a first direction, with a land between each adjacent pair of perforations in a row and with ribs between adjacent rows, the lands forming barriers to flow in the first direction across the plate and the ribs forming barriers to flow in a second direction across the plate, the second direction being normal to the first direction, the ribs having vents through a portion of their thickness, the vents extending from one side of a rib to the other side in the second direction, whereby flow channels are provided through the vents and the flow channels lying between each adjacent pair of lands provide a flow passageway to cross the plates in the second direction, the passageways in the group of plates being separated from passageways in any adjacent group of plates by one of the separator plates.
Although the group of plates may in fact contain only one perforated plate, there may be two or more perforated plates in the group of plates and in this embodiment adjacent perforated plates are aligned whereby the perforations of a row in one plate lie in correspondence with those of adjacent plates so that the lands and ribs of the plates lie in correspondence respectively with each other.
The invention will be more particularly described below with reference to embodiments in which the, or each, group of plates contains two or more perforated plates.
It will be appreciated that the flow passages can equally be provided in the first direction instead of the second direction, i.e. the lands are effectively the ribs containing the vents and the ribs are the lands.
The separator plates may be unperforated to provide complete separation of the passageways of the respective groups of plates. Alternatively, the separator plates may contain holes positioned and sized to provide controlled mixing of the fluids in those passageways. Such a separator plate will be referred to below as a mixing plate.
As indicated above each group of perforated plates preferably comprises at least two perforated plates but may contain three or more adjacent perforated plates as desired. A stack may, for example, comprise two or more groups of perforated plates separated by separator plates, each group containing two perforated plates having their perforations aligned in rows.
The passageways across the plates preferably traverse across the plates once only from a first edge to a second edge. However, in an alternative first specific embodiment, the passageways at one or both plate edges may be turned, e.g. by an appropriate passageway arrangement, through an angle whereby the passageway defined by the channels continues in a different direction through the stack, e.g. in the opposite direction so as to return from the second edge to the first edge.
In a second specific embodiment two or more separate passageways are provided across a group of plates whereby streams of different fluids may flow parallel to each other in the same layer provided by said group of plates. This embodiment can provide improved temperature profiles across the plates and reduced thermal stress.
Because the plates are stacked with the perforated plates of each group aligned with their perforations in rows, it will be appreciated that the solid regions (i.e. ribs and lands) of those plates between the rows of perforations and between the perforations are also aligned in rows. As the perforated plates, therefore, are stacked one above each other, the ribs and the lands are aligned through the stack and this provides strength through the assembled stack whereby the pressures generated in the bonding process can be withstood. The invention, therefore, provides a stack structure that can be bonded without the risk of the fins of the secondary plates collapsing under the pressures generated. The fins also provide the means of withstanding internal pressures in the operating streams. The rows of ribs and of lands may run in parallel lines across the plates but this is not essential.
The perforations may be of any desired shape but are preferably elongated slots.
The plates may be rectangular, square or circular for example or of any other preferred shape.
Where the plates are square or rectangular, each row of slots may extend from a first edge of the plate parallel to a second edge of the plate and for substantially the whole length of that second edge. It will be appreciated that a substantially unperforated edge or border will normally be required around the perimeter of the major faces of the plate to enable the plates of the stack to be bonded together and to provide pressure containment for the stream or streams. However, a completely unperforated border is not essential and slots in the border may be required for inlet and outlet means, for example. A plurality of rows of slots may, therefore, extend across the plate from the first edge to the opposite, third, edge.
Where the plates are circular the rows and passageways may extend from the outer perimeter as a segment of the circle towards the centre.
In one particular arrangement of the aforesaid second embodiment, a stack may be built up of one or more similar groups of plates, each group comprising an upper and a lower unperforated separator plate, a multi-passageway input layer in contact with one separator plate and a corresponding multi-passageway output layer in contact with the other separator plate, a centrally-disposed layer having at least one passageway for a first fluid and two or more transfer passageways for a fluid from each passageway of the input layer, a first auxiliary perforated plate lying between the input layer and the centally-disposed layer and a second auxiliary perforated plate lying between the output layer and the centrally-disposed layer, the perforations in the first auxiliary perforated plate being positioned to transfer fluid from each passageway of the input layer to the corresponding transfer passageways in the centrally-disposed layer and the perforations in the second auxiliary perforated plate being positioned to transfer fluid from the transfer passageways to the corresponding passageways of the output layer. The centrally-disposed layer can conveniently be formed of a plurality of main perforated plates as described above, as can the input and output layers.
The perforations or slots are preferably photochemically etched through the plates by known means, although spark erosion, punching or any other suitable means may be used, if desired.
The vents may be similarly formed and are preferably formed by photochemical etching. The vents are conveniently formed in the ribs on one surface of the plate so as to extend partially into the thickness of the rib (i.e. the thickness of the plate.). They may for example be of a depth equal to about one half of the plate. However, it may be advantageous to form vents in both surfaces of the plate, in which case the vents in one surface should preferably be staggered from those in the other surface.
For convenience the invention will hereafter be described with more specific reference to vents in the ribs although it will be appreciated, as indicated above, that they may equally be formed in the lands rather than the ribs.
A stack of adjacent perforated plates has rows of lands and rows of ribs. In the ribs between any adjacent pair of rows of lands there will be a plurality of vents forming flow channels across the plates. These flow channels together form a flow passageway that is separated from adjacent groups of flow channels, i.e. adjacent flow passageways, by the rows of lands. Thus each of the plurality of fluid channels forming an individual passageway may pass through the stack without any communication with the channels of another passageway. No mixing of fluid in those passageways can, therefore, take place and the stack functions purely as a heat exchanger with fluids at different temperatures passing through different groups of perforated plates or passing through different passageways in the same group of perforated plates.
In another embodiment of the invention there is provided intercommunication at selected positions between adjacent passageways. Thus cross-channels or cross vents may be etched or otherwise formed in the lands of the plates to provide access between adjacent passageways. The cross vents may be formed at any desired position along the passageways. Thus fluid flowing through separate passageways may be blended at pre-arranged positions on its journey through the passageways through the stack and this blending may be employed to ensure good fluid distribution and to improve heat exchange capability. (It will be appreciated that where the vents are in the lands rather than the ribs, then the cross vents will be in the ribs rather than the lands.).
Alternatively or additionally, inlets for a further fluid may be provided through the peripheral borders of the plates. Thus reactant may be introduced and mixed via the peripheral border inlets whereby the stack may be employed as a chemical reactor.
In another embodiment the invention provides a stack in which a fluid stream from one group of perforated plates may be injected into a fluid stream in an adjacent group of perforated plates. Injection holes for this purpose are provided in a mixing plate which separates the two groups of perforated plates. So-called xe2x80x9cprocess intensificationxe2x80x9d can be achieved by this means, and any reaction caused by the injection of a first fluid into a second fluid can be controlled by the pressure differential between the two streams, the size, numbers and spacing of the injection holes and by sandwiching the second stream between the first stream and a coolant or heating stream, as appropriate.
The density of the slots, and hence of the ribs or fins between each row of slots, may be varied, as required. Thus the number of slots per unit width or per unit length of a plate may be arranged to suit any particular flow/pressure drop/distribution change requirements.
The vents in adjacent pairs of ribs are preferably offset from each other so that fluid flow across the plates is continually changing direction in that it must follow a sinuous route. It will be appreciated that each time the flow passes through a vent, the flow area and hence velocity changes resulting in turbulence and good heat transfer through the mass of the plates, albeit with associated pressure drops. The skilled man of the art will, therefore, be able to design a wide variety of heat exchanger characteristics and to optimise the desired effects.
The vents may be formed normal to the direction of the rib or they may be angled through the rib so as to provide an increased sideways component of movement. The vents may be tapered, especially narrowed in the direction of flow to their exit into a slot. Thus flow velocity will increase as fluid enters a vent from a slot and will increase further due to the tapering effect.
It will also be appreciated that the main flow direction across the plates is through the vents and that flow normal to that direction, i.e. through any cross vents that are provided, will normally be restricted by the provision of fewer and/or smaller cross vents.
The rows of slots may extend linearly across the plate but this is not essential and they may be arranged in other desired patterns, e.g. herringbone or chevron.
The plates may be provided at their edges with extensions, e.g. in the form of lugs to assist location of the plates in a stack. Such lugs may be designed to be broken off after the stack has been assembled, e.g. by etching partway through their thickness along a line where the lug joins the plate. Alternatively the extensions may fit together in the stack to provide, e.g. one or more tanks on the side faces of the stack. Each extension may, for example, be in the form of a flat loop, e.g. of semi-circular profile, providing an aperture at the edge of the plate, the apertures of adjacent plates forming the volume of the tank when the plates are stacked together. The loops may be attached to the plate not only at their ends but also across the aperture by means of narrow ligaments. The tanks so formed can each feed fluid, e.g. process fluid, coolant or reactant which is fed into the tanks, into the channels of one passageway. Thus a tank will be coterminous on the side of the stack with the height and width of the passageway, i.e. a group of channels, to be fed. Where the stacks are polygonal in plan, a tank may be provided on one or more of the side faces of the stack. Where the stacks are circular in plan, a number of tanks may be spaced around the perimeter as desired.
Plates used to form the products of the invention may also be provided with a hole, e.g. disposed centrally through each plate, whereby a stack of the plates has a centrally-disposed discrete passageway for a fluid stream through the stack. To compensate for the loss of flow area where such a central hole is provided, it is possible, where the plate is provided with integral tank loops, to extend the plate outwardly between adjacent loops.
The plates of a stack are preferably of the same material and are preferably thin sheets of metal, e.g. of 0.5 mm thickness or less. The material is preferably stainless steel but other metals, e.g. aluminium, copper or titanium or alloys thereof, may be used.
Inlet and outlet headers or manifolds for the different fluids may be secured to the stack after bonding together of the stack plates or, alternatively, may be formed from integral features on the plates.
As indicated above, the components of a stack may be bonded together by diffusion bonding or by brazing. Diffusion bonding, where possible, may be preferred but, in the case of aluminium, which is difficult to diffusion bond, brazing may be necessary. It is then preferable to clad the aluminium surfaces, e.g. by hot-roll pressure bonding, with a suitable brazing alloy, in order to achieve satisfactory bonding by the brazing technique, although other means to provide the braze medium may be used, e.g. foil or vapour deposition.
The invention is particularly useful where it is desired to build up a large heat exchanger by bonding side by side a number of heat exchanger units. Each unit can be provided by a stack of plates of the invention. Each stack may, for illustration purposes only, be formed of plates of, say, 300 mm width by 1200 mm length and of the desired height depending on the thickness and number of plates. Several stacks can be placed side by side on a separator plate and then the assembly closed at the top by another separator plate. If six stacks, for example, are utilised side by side, a heat exchanger of about 1800 mm flow length is achieved. All required lugs, mitre sections, spacers, etc. can be formed integrally and built up from appropriate formations on each plate and all the stacks will be of the same height, being made up of identical plates. Such an arrangement has significant advantages in the manufacture of, for example, xe2x80x9ccryogenicxe2x80x9d aluminium heat exchangers, which conventionally have to be built up of layers of corrugations with separate side bars. Unless the height of the side bars relative to the height of the corrugations is correct lack of uniformity and unsatisfactory brazing of the product may result.
It is known that chemical reactions can be catalysed inside a structure such as a heat exchanger by providing a deposit of catalytic material in the internal passageways through which the fluid(s) to be catalysed are passed.
The perforated plates used in the present invention are particularly useful in this respect as the surfaces of the ribs, lands and vents can receive a catalytic material coating of relatively modest thickness and the slots in the perforated plate can receive a much thicker deposit of the catalytic material. Thus, for example, where the vents extend into the thickness of the ribs to a depth equal to about one half of the plate thickness, the catalyst deposit in the slots can be of depth up to half the plate thickness without causing any blockage of the vents.
In a further embodiment of the invention is provided a heat exchanger/catalytic reactor having a plurality of passageways to contain catalytic material to promote a chemical reaction in fluid(s) to be passed through those passageways, those passageways being separated by an intervening plate from a stack of one or more parallel perforated plates having a vented rib structure according to the present invention. Thus the stack of plates separated by the intervening plate from the adjacent passageways, which later will be filled with catalytic material, is formed from perforated plates, each having perforations arranged in rows across the plate in a first direction, with a land between each adjacent pair of perforations in a row and with ribs between adjacent rows, the lands forming barriers to flow in the first direction across the plate and the ribs forming barriers to flow in a second direction across the plate, the second direction being normal to the first direction, the ribs having vents through a portion of their thickness, the vents extending from one side of a rib to the other side in the second direction, whereby flow channels are provided through the vents and the flow channels lying between each adjacent pair of lands provide a flow passage to cross the plates in the second direction.
Once the heat exchanger structure has been completed and tested, the catalytic material may be packed into its passageways. However, the packing of the catalytic material will normally be completed immediately prior to the installation of the heat exchanger/reactor into its desired use position.
The passageways to contain the catalytic material are preferably defined between parallel ribs running the length of their plates to allow convenient introduction of the catalytic material and its subsequent removal at the end of its life cycle. The passageways may be closed off at one or both ends by a mesh to retain the catalytic material.
By means of this further embodiment, heating or cooling can very effectively be provided for the chemical reaction by passing a heating or cooling fluid through the stack of plates adjacent to the layers containing the catalyst. As indicated above, this structure causes such tortuous flow and turbulence that very good heat transfer properties can be achieved, especially with gaseous fluids. The catalysed reaction may, therefore, if exothermic, be effectively cooled by passage of a suitable cooling fluid, or if endothermic, may be heated and hence initiated or improved by passage of a suitable heating fluid, through the stack.
This further embodiment may also be used in conjunction with the above-described injection construction, i.e. the heat exchanger may have a first stack containing the passageways containing catalytic material, an adjacent second stack separated from the first stack by an intervening plate with injection holes and a third stack of the cooling or heating construction. The first stack may, for example, lie between the second and third stacks, or they may lie in the orderxe2x80x94first, second, third. Needless to say, these three stacks maybe repeated a number of times to form the complete heat exchanger/reactor.